IIO: ADC: fix return value check in stm32_dfsdm_adc_probe()

In case of error, the function devm_iio_device_alloc() returns NULL
pointer not ERR_PTR(). The IS_ERR() test in the return value check
should be replaced with NULL test.

Fixes: e2e6771c ("IIO: ADC: add STM32 DFSDM sigma delta ADC support")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>